1. Field of the Invention:
This invention relates to an ultrasonic observing apparatus for obtaining a tomographic image of an inspected object by an ultrasonic pulse echo method.
2. Related Art Statement:
There is already an ultrasonic probe for obtaining a tomographic image of a body interior from within a body cavity. The conventional ultrasonic probe has been mostly for a display purpose by a single scanning system such as linear scanning, radial scanning or sector scanning system.
However, when observing plaque or the like within a vein, it has been desired to three-dimensionally catch an image by simultaneously displaying a linear image and a radial image.
Therefore, so far a process such as is shown in FIG. 32(A) has been used wherein an ultrasonic probe 401 having an ultrasonic vibrator on one side is rotated by one round to be advanced and retreated to take several tomographic images which are processed to display linear images and radial images. In such a case, as shown, for example, in FIG. 33(A), the radial planes 1 to 8 are taken in by one scanning, only a fourth radial image such as the left side image of the displayed images shown in FIG. 33(B) is displayed, one scanning line in the radial planes 1 to 8 is sequentially taken in and such linear images as the right side images of the displayed image shown in FIG. 33(B) are displayed. By repeating such operations, radial images and linear images are displayed in one picture.
As such a apparatus whereby radial images and linear images can be obtained, an apparatus wherein an ultrasonic probe can make a radial scanning and can be moved in the axial direction is disclosed in the publication, for example, of Japanese Patent Application Laid Open No. 9439/1982 or Japanese Utility Model Application Laid Open No. 74108/1988. Also, an apparatus wherein an electronic scanning vibrator is made rotatable is disclosed in U.S. Pat. No. 4,543,960 and an apparatus wherein a vibrator is made rotatable for two axes is disclosed in EPC Patent Laid Open No. 0253268.
Also, as a scanning system for simultaneously displaying a radial image and a linear image, there is considered a spiral system wherein, as shown in FIG. 34, a probe 401 is advanced and retreated at any time and a probe is rotated with it. In such a case, there will be no problem with regard to the linear image but the scanning line of one radial image picture will be in the range x shown in FIG. 34(A) and the starting point and ending point will be different from each other. However, if the advancing and retreating speed is significantly lower than the rotating speed, there will be substantially no difference of the starting point and ending point and there will be no problem.
In such a scanning system as in FIG. 32(A) wherein radial images are taken in one by one as in the past, unless the probe 401 tip is positioned, that is, the starting point and ending point are accurately determined, the linear image will become low in resolution. The probe tip is desired to be stopped with the vibrating surface just above as, for example, in FIG. 32(B) but will be directed in an oblique direction by the inertia force as in FIG. 32(C). Then, in the case of displaying a linear image, the direction resolution will not be determined only by the ultrasonic vibrator 402 of the probe 401 but will depend on the positioning of the ultrasonic vibrator 402 and the resolution will deteriorate.
Also, as shown in FIGS. 34(a)-34(c), in the case of making a spiral scanning, as the rotary motor and advancing and retreating motor are independent of each other, they must be controlled in speed respectively individually. Then, in case the spiral scanning in FIG. 34(A) is made a reference, when the advancing and retreating speed against the rotation speed is increased to be higher than in the reference, it will be as in FIG. 34(B) but, when the advancing and retreating speed is reduced, it will be as in FIG. 34(C). Then, there will be disadvantages that the displaying surface will vary in the scanning position for a radial image will vary in the scanning range (resolution) for a linear image.
As characterisics of ultrasonic waves, the lower the frequency, the farther they will travel while the higher the frequency, the sooner the waves will be attenuated and the less distance they travel. Therefore, when the attenuation of ultrasonic waves is considered, in case an ultrasonic wave of a low frequency is used, it will be necessary to see it a long distance and the scanning speed had better be low. On the other hand, in case an ultrasonic wave of a high frequency is used, the scanning speed will be able to be increased. Therefore, in case the frequency of the ultrasonic wave is varied by replacing the ultrasonic vibrator 402, in order to obtain the optimum frame rate, it will be necessary to make the scanning speed corresponding to that frequency.
A technique of synchronizing the transmission of an ultrasonic wave and the rotation of an ultrasonic vibrator with each other is disclosed in the publication of Japanese Patent Application Laid Open No. 5134/1985. However, even if this technique is used, as the rotation and the advance or retreat of the ultrasonic vibrator are controlled independently of each other, the above described disadvantages will not be solved.
Also, in the conventional ultrasonic diagnosis, it is difficult to locate an affected part with one tomographic image. Therefore, particularly, in a single scanning type diagnosing apparatus, the diagnosis has been made by moving the position of an ultrasonic probe. In such a case, the ultrasonic probe has been manually moved, has been therefore difficult to position and has been likely to miss an affected part.
Further, moving the ultrasonic probe will give a pain to the examinee.
Also, in the apparatus shown in the publications of the above mentioned Japanese Patent Application Laid Open No. 9439/1982 and Japanese Utility Model Application No. 74108/1988, in order to obtain a linear scanned image, the rotary motion must be stopped, the rotary motion and advancing and retreating motion switching mechanism and positioning mechanism are required, the formation is complicated and the tomographic image of the desired part to be observed can not be quickly obtained.
There is also a method wherein data is stored in a memory and an image of any cross-section is obtained from the data in the memory. However, in such a case, many memories will be required and the circuit formation will be complicated.
Another apparatus, wherein one tomographic image of an inspected object and the other tomographic image intersecting with this tomographic image are displayed while showing the position relation of both so that various disease nests may be recognized as three-dimensional images, is disclosed in the publication of Japanese Patent Application Laid Open No. 143148/1981. This is to make three-dimensional a tomographic image by combining an electronic linear scanning and electronic sector scanning with each other.
However, in this apparatus, there are disadvantages that, in case it is necessary to change the positional relationship of the two tomographic images in order to obtain an objective tomographic image, the operator will have to manually operate them on the hand base side and a tomographic image of a desired part to be observed will not be able to be quickly obtained. Also, in the above mentioned apparatus, there is a disadvantage that, though the part in which the two tomographic images intersect with each other can be shown, the two tomographic images can not be made to intersect with each other to be displayed by automatically moving the scanning position to the desired part to be observed.
In such apparatus wherein both of a mechanical linear scanning and mechanical radial scanning can be worked as is disclosed in the above mentioned Japanese Utility Model Application Laid Open No. 7410/1988, there is a method wherein the mechanical linear scanning and mechanical radial scanning are simultaneously made, three-dimensional data is taken in and is stored in a memory as described above and an image of any cross-section is obtained from the data in the memory. However, there are disadvantages that, in such a case, it will take time to display the two tomographic images and also to select tomographic image to be observed.